So this idea just kinda struck me and I wanted to check if anything too much like it already existed.

Basically, the idea is this: an entity that attaches itself to children and becomes what parents describe as an "imaginary friend." Only prepubescent children can see it and they all tend to perceive it slightly differently, but it remains largely the same (haven't decided on what it should look like). The entity will then proceed to "pull pranks" on adults, which will escalate from the usual benign stuff up to dangerous and then lethal.